1. Field of the Invention
This invention relates to methods of cold forming, more particularly, backward and forward extrusion of ferrous metal workpieces using as a lubricant a coating composition of a film-forming, chlorine-containing polymer and a destabilizing agent, said coating being deposited from a liquid composition directly on the metal surface to form an essentially integral film thereon.
2. Description of the Prior Art
The use of lubricant materials as surface treatment for metals in cold forming, including backward and forward extrusion procedures, is well known in the art. The simplest known methods involve the use of ordinary lubricating oils which have been utilized on various metal materials in die-forming and drawing procedures for a substantial period of time. Lubricating oils, however, have one drawback and that is they fail to provide satisfactory performance under extreme pressure conditions, especially as are encountered in the forming of harder metals such as steel with the result that the failure of the lubricant under these conditions results in scoring of the metal during the forming step. It is believed that this failure of the lubricants under these high-stress conditions is attributable to the squeezing out of the lubricant from between the work and the die under the high pressures used. Improved phosphate coating processes for these so-called impact extrusions, specifically relating to mild steels, were developed in the 1930's. These traditional processes, still widely used, employ a phosphate (zinc, iron, maganese) coating chemically applied to the surface of the workpiece or blank. The phosphate coating served a dual purpose, that of a separating layer and partial lubricant and as a lubricant absorbent and carrier. The lubricants employed and still in current application were soap such as sodium stearate soaps and other additives such as graphite or other extreme pressure lubricant additives. For other applications, compositions have been employed which contain pigment type additives which may be generally described as infusible. These pigments are intended to separate the die and the workpiece at the points of extreme deformation when the pressure or temperature during the drawing or forming process is too great to be withstood by conventional lubricating materials. Examples of such pigment additives are materials such as clay, lime, calcium carbonate, molybdenum disulfide, titanium dioxide and graphite. In this practice there is thus provided a dry lubricant composition which primarily consists of a high pressure lubricant material such as the insoluble or infusible pigment described above. For more severe application, this pigment technology has been added to the phosphate coating so that typical lubricant systems would consist of phosphate coatings, soap films and an infusible pigment such as molybdenum disulfide. These compositions and procedures are described in U.S. Pat. No. Re. 24,017.
The method described in the aforenoted Reissue Patent involves three basic co-acting factors which include formation of on integral coating directly on the work, application of an organic binder coating on the integral coating, said organic binder containing a dispersion of fusible pigments. The integral coating formed on the ferrous metal workpieces is brought about by electrochemical reaction of the iron with reactive materials to form chemical coatings such as iron sulfide, iron phosphate, iron oxalate, or iron fluoride. The organic binder material employed may include various synthetic and natural resins such as acrylics, alkyl resins, cellulose nitrate polymers, asphaltum, shellac, polyvinyl chloride, polyvinyl acetate, and styrene polymers and the like. The fusible pigments employed are those which have a Moh hardness of less than 5 and melt below the melting point of the work or the die, whichever is lower. The melting range is described as generally above 500.degree. C. Examples of fusible pigments include aluminum stearate, antimony oxide, copper powder, lead borate, sulfur, etc.
In the process described in the U.S. Pat. No. Re. 24,017, the ferrous metal workpiece is provided with an integral, chemically bonded coating (i.e., ferrous sulfide) formed on the surface thereof which is then further coated with a composition of a fusible organic resin binder containing admixed therein an inorganic, fusible solid material as a secondary or high or extreme pressure lubricant. However, the phosphate methods are expensive and cumbersome to employ since the described procedures involve a chemical treatment of the metal surface which is difficult to control due to normal acid bath depletion, and the subsequent application of an organic coating represents a separate coating and handling operation.
More recently, organic polymers have been employed as the lubricant in the drawing of metals, particularly mild steel workpieces. Polymers which have been considered include polymethylmethacrylate polymer, polyethylene, polypropylene, polyvinyl chloride and nylon in solvent solutions. These procedures are described in Sheet Metal Industries, July, 1963. Solvents obviously present a toxicity and a flammability hazard.
In Sheet Metal Industries, October, 1967, Rao also describes the use of polyethylene as a lubricant in the deep drawing of workpieces. The application of the polyethylene to the workpiece was by a variety of procedures, including hot-dip, adhesives, cold spraying, flame spraying, extrusion coating, emulsion coats, and solution coating from solids.
Blake, et al. in Metallurgia and Metal Forming, January, 1972, pp. 30 and 31, disclose the attempted use as lubricants of polyvinyl chloride films laid down from solvent systems. This procedure, however, did not give satisfactory results.
While these prior art procedures appear to have functioned satisfactorily in many respects, they do not produce the desired results under all conditions, especially the severe conditions encountered in backward and forward extrusion of metal and, more paticularly, steel workpieces.